In this technological world, people are starting to worry about the effects of radiation and electromagnetic waves on living tissue. A study was conducted that examined pregnant women's exposure to various electromagnetic waves and the incidence of mutations that resulted in birth defects. Scientists studied the effects of the following ER waves: UV, infrared, microwaves, radio waves, X-rays, and visible light. According to the study, only X-rays produced a noticeable increase in mutations that resulted in birth defects.

Which letter MOST likely represents X-rays?